Reminder: ammo/gunpowder crafting does not become available until the archetype is taken at lv3, hence the large starting ammunition stated in extras.
When Crafting Black Powder: You follow the same rules as stated for crafting Non magic items in the player handbook, but the crafting time is halved. I.e. To craft 4lb of black powder (Market Value 70gp) it would take 35gp and a week of crafting. - You may be able to purchase BP at rare locations: You can buy a Horn of 2lb of Black Powder for around 35GP, or a Keg of 20lb of Black Powder for around 250GP. - Alternatively to save time, just start with an artificer’s bag that produces black powder.
When Crafting Ammo: Using your Tinker’s tools you may spend money to collect raw materials for crafting ammunition. Which weigh 4 pounds for every 50GP spent. As part of a long rest, you can use tinker’s tools to craft a single batch of one ammunition type. Then subtract half the market value of the created ammunition from the total GP worth of raw materials you are carrying. Batch sizes and prices are shown in the Archetype's firearms table. At the expense of paying the full market value, the crafted batch produced will be silvered (cannot be done for explosive ammunition). When crafting Bad News or Mortar ammo, be sure to subtract the gunpowder when the ammo is crafted, and not when it is shot. However this gunpowder does not count towards the crafting cost of the ammo.
Gun Drafting: Before you can craft a Firearm you must first create a draft of it. During a Short Rest, you can choose a Firearm and then make a tinkering (Intelligence) check, if you meet or succeed the DC of the Firearm; you successfully create a draft for the Firearm, and can now start to craft that Firearm. However, you cannot attempt to draft a Firearm if you are currently in the process of crafting another item. How often you can attempt to draft a firearm is up to the DM’s discretion.
Crafting a Firearm: When crafting a Firearm you follow the same rules as stated for crafting Non magic items in the player handbook, but the normal crafting time is halved. I.e. a Pepperbox (Market Value 250GP) would only cost 125GP and 25 days to make. Additionally a day’s crafting can be done as part of a long rest but a tinkering check must be made to determine any success.
Extras: - At lv1 Fighter you can start with a Musket, 60 Musket Rounds and 2 Horns of BP, instead of a bow and 20 arrows. Additionally the Pistol, and Palm Pistol (both with 60 Rounds of respective ammo) replace the light crossbow/2 hand axes as the two options. Fixing after a misfire (in combat): Wisdom tinkering check.
Amount of black powder per shot (1 BP charge is 0.01lb of BP): Palm pistol, Pistol, & Pepperbox use: 1 BP charge per shot. Musket uses: 2 BP charges per shot. Blunderbuss uses: 3 BP charges per shot. Bad News uses: 3 BP charges per shot (consumed during ammo crafting, not when ammo is shot). Mortar uses 40 BP charges per shot (consumed during ammo crafting, not when ammo is shot).
Weight per ammo round for every gun: Palm pistol: 0.015lb Pistol & Pepperbox (Primary): 0.025lb The Pistol and Pepperbox share the same ammo type. Musket: 0.03lb (50cal) Blunderbuss: 0.05lb Bad New: 0.2lb Mortar: 0.8lb